This invention relates to the correction of various aberrations of a wavefront of light over a relatively wide band of wavelengths. In many modern optical systems, it is desirable to change or correct the shape or structure of a wavefront of light. Transmission of light through an optical system often distorts the wavefront in a number of different ways. These distortions alter information carried by the wavefront in a wavefront analysis system and render the wavefront unusable.
A particular application of wavefront analysis relates to the alignment of mirror segments in a segmented mirror system. A segmented mirror system replaces a monolithic primary mirror as used in large aperture telescopes. Such segmented mirrors are becoming more popular for both astronomical and military applications. Segmented mirror systems use arrays of relatively small optical segments, each of which must be actively controlled to allow the composite optical surface to maintain the desired shape of an ideal monolithic surface.
In such systems, it is not only necessary to control the tilt of a mirror segment along two axes, but the displacement of the segment along the direction perpendicular to the segment surface must be controlled also. The three degrees of control per segment allow two or more segments to reflect light from a given source to the same geometric point with the same phase, so that the light behaves as if it were reflected from a single ideal surface.
The errors in tilt are easily handled, since they can be readily observed in the far field of the telescope. The displacement errors are not so readily observed, however, since they are normal to any image surface and must be measured to a fraction of the wavelength of light. Interferometers are used to measure displacement errors between different mirror segments by interfering different portions of a wavefront reflected from the entire mirror surface. When two portions of a wavefront are combined which correspond to reflections from two misphased mirror segments, destructive interference occurs which is detectable in the interferometer. Due to the wave nature of light, the interference pattern repeats as the mirror separation is increased beyond one half wavelength. A one half wavelength mirror separation corresponds to a path length difference of one full wavelength since the extra distance is travelled both before and after reflection. The one wavelength path length difference causes a relative phase shift of 2.pi., which puts the combined waves back in phase. A traditional interferometer cannot discern between a phase shift .phi. and a phase shift of .phi.+2.pi. when used with a monochromatic source because the interference pattern repeats. Because of this so called "2.pi. ambiguity factor", an interferometer alone is not sufficient to accurately detect the displacement error in monochromatic light. Alternative position sensing means must therefore be employed.
Interferometers are generally designed to function with a collimated beam or a spherical wavefront. The segmented mirror to be tested is often of a parabolic or near parabolic shape. A spherical wavefront of light used to test the segmented mirror will undergo distortion as it reflects off the parabolic segmented mirror. This distortion is called spherical aberration and is due to the reflection of a spherical wavefront off a non-spherical surface. Because the light incident on the mirror surface is not in a direction perpendicular to the surface, it is not reflected back upon itself as a spherical wavefront. Furthermore because the difference between a spherical surface and a parabolic surface increases with radius, the spherical aberration increases toward the edges of the wavefront. The spherical aberration thus causes different portions of the wavefront to focus at different points along the optical axis.
The correction of spherical aberration due to reflection from a paraboloidal reflecting surface is most easily accomplished by introducing into the optical path of the system a correction device known as a null lens. A null lens introduces aberration to the wavefront passing through it which compensates for the aberration introduced by the non-spherical surface. Thus a spherical wavefront passing through the system is corrected for spherical aberration, and leaves the system as if it had been reflected off a spherical surface.
Traditionally, null lenses have been purely refractive devices because for large mirrors a reflective device would require one that was too large and expensive to be practical. Refractive elements have the undesirable feature of dispersion. As a result, nulling occurs only within a very narrow band of wavelengths. This is satisfactory for measuring the continuous errors that afflict the surface figure of a monolithic mirror, but cannot be used to measure displacement errors larger than one half a wavelength of the light used. The 2.pi. ambiguity factor in displacement measurements greater than one half wavelength makes it impossible to get an accurate interferometric measurement. This displacement may be determined, however, if the wavelength of the light incident upon the reflecting surface is varied while output measurements are taken with the interferometer over a continuous range of wavelengths. However, this technique requires a null lens which is functional over a band of wavelengths much wider than that which has been achieved with an all-refractive null lens.